Custom Text based Diesel type for an enum

state.rs

#[derive(Serialize, Deserialize, Debug, PartialEq, Eq)]                                                                        
#[serde(rename_all="snake_case")]
pub enum State {
    Pending,
    Sending,
    Sent,
    Failed,
}

impl fmt::Display for State {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", match *self {
            State::Pending => "pending",
            State::Sending => "sending",
            State::Sent => "sent",
            State::Failed => "failed",
        })
    }
}

impl FromSqlRow<Text, Pg> for State {
    fn build_from_row<R: Row<Pg>>(row: &mut R) -> Result<Self, Box<Error+Send+Sync>> {
        match String::build_from_row(row)?.as_ref() {
            "pending" => Ok(State::Pending),
            "sending" => Ok(State::Sending),
            "sent" => Ok(State::Sent),
            "failed" => Ok(State::Failed),
            v => Err(format!("Unknown value {} for State found", v).into()),
        }
    }
}

impl AsExpression<Text> for State {
    type Expression = AsExprOf<String, Text>;
    fn as_expression(self) -> Self::Expression {
        <String as AsExpression<Text>>::as_expression(self.to_string())
    }
}

impl<'a> AsExpression<Text> for &'a State {
    type Expression = AsExprOf<String, Text>;
    fn as_expression(self) -> Self::Expression {
        <String as AsExpression<Text>>::as_expression(self.to_string())
    }
}